The present invention relates to signal conversions, and, more particularly, to analog/digital (A/D) converters.
Analog signals have to be converted to digital signals in many kinds of electronic equipment. A significant example of this type of equipment is provided by digital cellular telephones. In these applications, the requirements of low consumption and high performance impose particularly severe constraints on the design of the integrated circuits which form the electronic system of the equipment.
One type of A/D converter which is particularly advantageous for use in these electronic systems will be generally described below, and which makes use of binary code weighted capacitors with associated switches which are controlled by a logic circuit by a process known as SAR (Successive Approximations Register). To obtain a large dynamic field and a good immunity to noise originating either within or outside the integrated circuit, the circuit portions for analog processing of the signal are preferably formed from structures of the fully differential type.
In certain cases, however, analog signals which are not of the differential type, i.e. in the form of variations of voltage of opposite sign to a common reference potential, but are of the asymmetric or xe2x80x9csingle-endedxe2x80x9d type, i.e. in the form of a single voltage variable with respect to a reference potential, are present and have to be converted into digital form. In these cases, there are two alternatives: either converters operating with single-ended input signals are used, or the signal is converted to a differential signal and then applied to a converter which operates with differential input signals. In the first case, it is necessary to make arrangements, where possible, to overcome the negative effects of noise and other limitations intrinsic to the processing of a single-ended signal, and in the second case it is necessary to allow for a higher power consumption and other undesired effects (distortion and offset) of the circuit to be connected before the converter.
An object of the present invention is to provide an A/D converter which carries out the conversion of a single-ended signal directly, and which has a noise immunity at least equal to that of a converter for differential signals.
This object is achieved according to the invention by an A/D converter having a set of sampling capacitors weighted according to a binary code and charged through a first capacitive unit, whose capacitance is equal to the sum of the capacitances of the set, at a voltage Vcmxe2x88x92Vin/2. The conversion is carried out by an SAR process by a comparator and a logic unit which operates the switches associated with the capacitors. The final position of the switches is loaded into a register which supplies the digital output signal. To prevent any disturbances in the power supply and reference potential sources from affecting the accuracy of the conversion, two further capacitive units and are provided. These have the same capacitance as the first capacitive unit, and these make it possible to present all the disturbances at the input of the comparator in common mode and therefore without any effect on the output.